Agonists acting at the 5 opioid receptor (MOR) (e.g. morphine and its analogs) are the mainstay of pain management;however there are serious adverse effects (e.g. dependence) and social and legal issues which limit their use. Consequently there has been considerable interest in the peripheral analgesic effects of opioids. In addition to MOR, the 4 opioid receptor (DOR) is an attractive target for drug action since there are fewer adverse effects than with MOR activation. However, in general, the efficacy of 4 agonists to promote analgesia is weak to moderate. Our preliminary data suggest that the affinity and/or efficacy of DOR agonists can be regulated by the k opioid receptor (KOR) antagonist, nor-BNI. In this R21 exploratory application, we propose to study the regulation of DOR systems by k opioid antagonists in primary cultures of rat trigeminal ganglion neurons and in behavioral models for analgesia. Our specific aims are: 1) To examine the effect of KOR antagonists on the efficacy and potency of selected DOR agonists. In this aim, we will determine the effect of the KOR antagonist, nor-BNI, on concentration-response curves to selected DOR agonists in primary cultures of rat sensory neurons of the trigeminal ganglion. We will also examine the effect of different KOR antagonists on the concentration-response curve to the DOR agonists DPDPE and DADLE. 2) To assess the mechanism by which KOR antagonists regulate DOR agonist responsiveness. Our preliminary data indicate that KOR occupancy with nor-BNI increases or decreases the potency of DOR agonists (DPDPE and DADLE, respectively). In this aim we will test the hypothesis that the effect of nor-BNI is due to a change in agonist affinity for DOR using competition binding experiments. We will also determine if the effect of nor-BNI is mediated by KOR using siRNA knock-down of KOR. 3) To investigate the effect of KOR antagonists on responsiveness to DOR agonists in a behavioral assay of thermal and mechanical allodynia. This aim provides a translational extension of the foundation work of Aims 1-2. We will determine the effect of occupancy of KOR with the antagonist nor-BNI on concentration-response curves to a series of DOR agonists in a behavioral assay of peripheral analgesia. We will also examine the effect of different KOR antagonists on the concentration-response curve to the DOR agonist DPDPE. If our preliminary results are substantiated, a new pharmacological approach for pain management with improved therapeutic efficacy and increased selectivity may be developed. PUBLIC HEALTH RELEVANCE: The 4 opioid receptor (DOR) in the periphery is an attractive target for analgesic drugs, however, in general, the efficacy of 4 opioid receptor agonists to promote analgesia by acting in the periphery is weak to moderate and variable. In this R21 exploratory application, we propose to study the regulation of DOR systems by k opioid antagonists in primary cultures of rat trigeminal ganglion neurons and in behavioral models for analgesia. The results obtained will provide a framework for a more comprehensive study (RO1) of the regulation of DOR agonist efficacy in nociceptors and may lead to the development of a new pharmacological approach for pain management with improved therapeutic efficacy and increased selectivity.